The adhesive interactions between cells and between cells and the extracellular matrix are believed to play critical roles in a wide variety of processes including, for example, modulation of the immune system, regulation of developmental processes and tumor progression and metastasis. These interactions are mediated by adhesion molecules which transduce information from the extracellular to the intracellular matrix.
Four families of adhesion molecules which mediate these interactions have been identified: the integrins, the cadherins, the selecting, and immunoglobulin-related molecules. In general, adhesion molecules are transmembrane proteins which contain an extracellular domain for interacting with an extracellular matrix or cellular component, a transmembrane domain spanning the cell membrane and a cytoplasmic domain for interacting with one or more cytoskeletal or cytoplasmic components.
The cadherins play an important role in the establishment and maintenance of intercellular connections between cells of the same type (reviewed in Geiger B. et al. (1992) Annual Review of Cell Biology 8:307; Kemler R. (1993) Trends in Gastroenterology 9:317; Takeichi M. (1990) Annual Review of Biochem. 59:237; Takeichi M. (1991) Science 251:1451). Cadherins are a superfamily of structurally related molecules that function in Ca+2-dependent homophilic adhesion. Cadherins are expressed on cells that form solid tissues, and are responsible for segregating and sorting cells during embryogenesis, establishing cell polarity, and maintaining tissue morphology. Structurally, cadherins are single chain polypeptides that are synthesized as precursors and cleaved during post-translational processing. They have large extracellular regions made up of 5 homologous domains, a single transmembrane segment and a cytoplasmic tail.
The cadherins are synthesized as precursors that are cleaved during post-translational processing. The mature cadherins are single chain molecules which include a relatively large extracellular domain (typically divided into five sections or “ectodomains”), a single transmembrane region and a cytoplasmic tail. Among the classical cadherins (i.e., P-(placenta), E-(epithelial), and N-(neural) cadherin), the cytoplasmic domain contains the highest degree of homology. The high degree of homology observed for the cytoplasmic domain reportedly is a reflection of the association of cadherins with a group of intracellular proteins, called catenins, that stabilize cadherin active conformation (Kemler R. (1993) Trends in Gastroenterology 9:317). It is generally believed that sequences in the extracellular domain are necessary to mediate homophilic (i.e., cadherin-to-cadherin) binding. A review of the literature indicates that research directed to understanding cadherin-mediated adhesion has focussed on efforts to elucidate the mechanism underlying cadherin-mediated homophilic cell adhesion. Little attention has been directed to understanding what, if any, role is played by cadherins in heterophilic adhesion. While it has been known for some time that integrins and other adhesion molecules function in immune system modulation, e.g., by playing a role in the adhesion of peripheral lymphocytes to endothelium and in homing to lymph nodes, relatively little is known regarding the mechanism by which lymphocytes home and transmigrate through the vascular endothelium to specifically target certain tissue locations, such as the synovium.
The most highly conserved sequence shared by cadherins lies within the cytoplasmic domain. It is this region that mediates interaction with the cytoplasmic catenins proteins (Hirano S. et al. Cell 70:293-301, 1992). The presence of the cytoplasmic domain is essential to functioning of the cadherin as deletions in this region abolish catenin binding as well as cell-to-cell adhesion (Hulsken J, et al. J Cell Biol 127:1375-80, 1994). The catenins (α, 102 kDa; β, 88-93 kDa, and γ, 80-83 kDa) begin to associate with cadherins almost immediately upon biosynthesis in a stable manner that is not disrupted in TX-100 detergent (Takeichi M. Curr Opin Cell Biol 7:619-27, 1995) and are thought to mediate anchorage of cadherins to the cytoskeleton (Yap A S. et al. Annu Rev Cell Dev Biol 13:11946, 1997). Functionally, α-catenin is necessary for cadherin mediated homophilic adhesion. Tumor cells expressing E-cadherin at the cell surface, but lacking α-catenin expression, fail to form cell-to-cell contacts unless α-catenin expression is restored through transfection (Chen H. et al. J Cell Sci 1141345-56, 1997; and Knudsen K A. et al. J Cell Biol 130:67-77, 1995). β-catenin is homologous to the Drosophila segment polarity protein armadillo as well as the cadherin associated protein plakoglobin. Plakoglobin, also termed γ-catenin, interacts more weakly with the cadherin/catenin complex, and is not always seen in cadherin precipitates.
A number of newly identified cadherin cDNA clones have been isolated using consensus oligonucleotides corresponding to cytoplasmic domain sequences that are highly conserved among cadherins and PCR cloning (Suzuki S. et al. Cell Reg 2:261-70, 1991).
Although cadherin function classically involves homophilic cell-to-cell adhesion (i.e., E-cadherin binds E-cadherin typically on another cell of the same type), however, murine E-cadherin expressed on epidermal keratinocytes also mediates adhesion to E-cadherin of Langerhans cells (Tang A. et al. Nature 361:82-5, 1993). Recently, we identified another counter-receptor for E-cadherin, namely the integrin αEβ7 which is expressed on intraepithelial T cells (Cepek K L. et al. Nature 372: 190-3, 1994 and U.S. Pat. No. 5,610,281). This represents an example of heterophilic binding of E-cadherin to the αEβ7 integrin counter-receptor.
Rheumatoid arthritis (RA) is a chronic systemic autoimmune disease of unknown etiology that affects 1-2% of the population. It is dominated by progressive joint destruction that can result in marked disability. Histologically, the joints reveal synovial hyperplasia mainly of type A synoviocytes but also of type B synoviocytes, lymphocellular infiltration of T and B cells, and neovascularization. These processes lead to the secretion of destructive factors and invasive growth of the synovial membrane (pannus) into the adjacent cartilage and bone with the consequent destruction of the joint.